This work focuses on the development of the cerebral cortex using neuroanatomical techniques to study developing axonal connections in mice, rats and non-human primates. We have concentrated on the corticospinal and the callosal projections emphasizing the role played by collateral elimination in the development of cortical connections. Experiments using transplants of rat cerebral cortex during development have identified position within the tangential plane of the cortex as a critical factor in determining which of the initially extended projections, cortical neurons will maintain. However, our observations on forelimb placing behavior in such preparations indicate that animals with homotopic transplants into the sensorimotor cortex show substantial behavioral sparing, but that animals with hetertopic transplants do not. In some tasks the animals with heterotopic transplants showed even a greater impairment than lesioned animals without transplants. These results suggest that despite their seemingly appropriate connections, heterotopic transplants are not able to become fully functionally integrated within the circuitry underlying forelimb placing behaviors. Experiments using retrograde tracers to label corticospinal or callosally-projecting neurons in dysmylinated jimpy mutant mice show that the tangential distributions of these two projection systems undergo a developmental restriction similar to that seen in normal animals. This indicates that maturation of function accompanying myelination is not critical to the normally occurring collateral elimination within these cortical projection systems. Preliminary experiments using anterograde tracers to label corticospinal axons in infant rhesus monkeys indicate that the distribution of this projection within the spinal cord matures postnatally. There is little indication in infant monkeys for the direct corticospinal projection onto motoneuronal pools that is characteristic of the projection from the precentral gyrus in adult animals.